There is a book out by Barbara Ehrenreich, called Bright-Sided: How the Relentless promotion of Positive Thinking Has Undermined America. While I do believe that a positive outlook on life is worthwhile, I sympathize with her criticism.
Once, I was talking with my grandmother, who was then 95 years old and who died a few years ago. She told me that she found getting older not always easy. For instance, she struggled with some physical discomforts and with doubts and feelings of guild with regard to religion. She had talked about her discomfort with her doctor who had said to her: "You have got to remain positive madam!" My grandmother told me how unpleasant she had found this. "Many people tell me to think positively. But it is very unpleasant when people say that. That won't make me feel any better. If anything, it makes things worse."
Like I said, I actually do believe in the value of positive thinking but trying to convince others to think positively I have not often seen to work. You run the risk of making them feel you don't take them seriously, like they are really exaggerating and should not make such a fuss of their problems. And apart from that, is not easy to think positively when instructed to. That would be like saying to an inhibited person: be spontaneous! My experience is that works better to take seriously what people tell you. When they say they have a problem, acknowledge that and try to help them find a way to cope with it and to take small steps forward if possible.